1. Field of the Invention
The field of the invention is ring lasers, more particularly ring laser gyroscopes, including multioscillators, and the methods and apparatus for stimulating ring lasers. Its classification is expected to be in class 356.
2. Description of Related Art
In this description, the terms wavelength, quarter wavelength, half wavelength, and similar words, are measured at the excitation frequency of the ring laser.
Over the past twenty five years the gaseous medium planar ring laser gyroscope has been developed and evolved as a reliable and relatively environmentally insensitive inertial rotation sensor. Planar ring laser gyroscopes of both triangular and square geometries have been used regularly in inertial navigation systems and flight control systems in both commercial and military aircraft. The primary advantage of the ring laser gyroscope over the spinning-wheel mechanical gyroscope is that it can be configured into a truly strapdown system. It can not only have a much larger dynamic range than its mechanical equivalent but also be free of the disadvantages of mechanical bearings. Its ability to withstand relatively large mechanical shock is enhanced without permanently degrading its performance. Because of this and other features, the expected mean time between failures of ring laser gyroscopes in inertial navigation systems is several times longer than the spinning wheel mechanical gyroscopes it replaces.
At low measured rotation rates, the retroscatter from the mirrors of a conventional planar ring laser couples energy from one of the oscillating beams traveling in one direction into the beam propagating in the other direction around the ring laser. The frequencies of the two beams are locked together, yielding zero rotation information. To avoid locking of the frequencies, it is conventional for ring laser gyroscopes having a planar configuration to use mechanical rotational dithering schemes to rock the gyro about its sensitive axis. The information from the gyro can then be processed to avoid the effects of the frequency locking, or "lock-in." Mechanical dithering is very effective to reduce the effects of lock-in, and it makes the ring laser gyroscope a successful navigation-grade gyroscope. However the mechanical dithering in such a ring laser gyroscope adds a quasi-random noise component to the gyro output signal that limits its performance Additional costly apparatus or techniques are used to minimize the noise produced by the dithering.
For certain military or space borne uses and for systems needing fast updates of position, body or mirror dither cannot be tolerated.
Alternative biasing techniques have been developed using a nonreciprocal Faraday effect created either by placing an axial magnetic field directly in the gain medium of the laser or by placing a solid glass element within the ring laser cavity. The differential phase shift produced between the counterpropagating beams separates their lasing frequencies and shifts their lock-in band to input rates higher than those being measued. An example of this technique, used in a configuration called a multioscillator, may be found in U.S. Pat. No. 4,818,087 entitled, "ORTHOHEDRAL RING LASER GYRO," which issued on Apr. 4, 1989 to Terry A Dorschner.
More recent multioscillator gyroscopes use a nonplanar light path both to force the lasing polarizations to be substantially circularly polarized and to create a frequency difference, known as the reciprocal splitting, between the two polarities of circularly polarized light. The beam pairs of each polarization operate independently without coupling with one another to create two laser gyroscopes operating in the same cavity. When a Faraday elements is added, with a properly applied magnetic field, to the cavity, the frequencies of the counterpropagating beam pairs for both gyros are separated by an equal but opposite amount. The difference of the output beats of the two gyros is substantially independent of the size of the nonreciprocal Faraday bias, and it is doubly sensitive to rotation.
In such a device there are at least four lasing modes having: a left-circularly polarized anticlockwise frequency (La), a left circularly polarized clockwise frequency (Lc), a right-circularly polarized clockwise beam (Rc), and a right-circularly polarized anticlockwise beam (Ra). The nonreciprocal Faraday splitting between the clockwise and anticlockwise beams is typically of the order of one mega-hertz. At least four mirrors form the ring resonator path which is enclosed with an excited laser medium within a closed ring cavity to cause the four beam modes to lase.
One of the mirrors is semitransparent to allow light to leave the resonator and fall upon a photo detector for signal processing. Demodulator circuitry removes the Faraday bias carrier frequency and leaves a beat signal doubly sensitive to rotation compared with the equivalently sized planar dithered Ring Laser Gyro.
A more modern form of a multioscillator-type ring laser gyroscope is described below in the description of Parent 3.
The published prior art teaches magnetic fields positioned around the ring laser bore to separate the frequencies of multiple laser beams. For example, an article entitled, "Properties of Zeeman Multioscillator Ring Laser Gyro," by V. Sanders, S. Madan, W. Chow and M. Scully, published in the 1979 Proceedings of the IEEE; U.S. Pat. No. 4,213,705 which issued Jul. 22, 1980 to Virgil E. Sanders for a FOUR MODE ZEEMAN LASER GYROSCOPE WITH MINIMUM HOLE BURNING COMPETITION.; and U.S. Pat. No. 4,475,199 which issued Oct. 2, 1984 to Virgil E. Sanders et al. for a ZEEMAN MULTIOSCILLATOR RING LASER GYRO INSENSITIVE TO MAGNETIC FIELD AND DETUNING FREQUENCIES teach a multioscillator non-planar ring laser gyro having counterpropagating right and left circularly polarized ring laser beams. A constant magnetic field, produced by a D.C. coil, is placed around the bore of the ring laser to produce non-reciprocal frequency splitting to separate the frequencies of two of the beams from the frequencies of the other two beams. The magnetic field is a constant field, and it does not excite the laser gas. The devices of those references are energized by electrical discharge, not by electro-magnetic fields.
U.S. Pat. No. 4,616,929 which issued Oct. 14, 1986 to Bernard Bernelin, et al. for a COMPACT, INTEGRAL, 6-MIRROR TRIAXIAL, LASER RATE GYRO teaches an octahedral ring laser gyro which also is energized by electrical discharge between the cathode 14 and three anodes 18, 19 and 20.
An article which appeared on page 132 of Volume 19, Number 5, Sep. 1, 1971, of Applied Physics Letters about a, "Waveguide Gas Laser," by P. W. Smith teaches a linear laser with electrical excitation between an anode and a cathode. Three in-line cross-hatched elements, one of which is grounded, are shown adjacent the laser bore and attached to an, "RF Exciter" The two outer elements are shown energized in phase. The elements and their separation are not mentioned in the article, but they are probably electrodes.
U.S. Pat. No. 3,772,611 which issued Nov. 13, 1973 to P. W. Smith for a, "Waveguide Gas Laser Device," teaches a continuous single coil receiving r.f. energy to excite a ring laser gyro, but the position of the excitation region appears free to wander. Also, being a mirrorless device, there would be considerable scatter and uncontrollable modes produced. This patent does not describe a ring laser gyroscope.
U.S. Pat. No. 3,873,884 which issued Mar. 25, 1975 to F. C. Gabriel for an, "Electrodeless Discharge Lamp and Power Coupler Therefor," teaches that a gas discharge lamp may be excited using an r.f. field. The recitation of "stability" of operation of the Gabriel lamp refers to "reliability" of the lamp, not stability in positioning the excitation region of the lamp.
W. W. Macalpine, et al., in an article published in the December 1959 Proceedings of the IRE describes, "Coaxial Resonators with Helical Inner Conductor," describes certain constructions of r.f. coils.